In many manufacturing environments, workpieces are supplied in bins. It is a common industrial problem to load machines with such pieces. Often human labor is employed to load an otherwise automatic machine with workpieces. Such jobs are monotonous and do not enrich human life. Young people today have increasingly higher aspirations for good jobs. While the cost of labor is increasing, human performance remains essentially constant. Furthermore, the enviroment in manufacturing areas is generally unhealthy and, when workpieces are inserted into machines, limbs are often exposed to danger. These factors strongly suggest that alternatives to the use of human labor be considered.
The engineering alternatives are the use of mechanical feeders, the preservation of orientation of parts throughout the manufacturing process, and the use of robots with vision to feed pieces from bins. To be viable, any approach must be competitive on an economic basis. The advantages of using robots with vision can be appreciated when the disadvantages of the alternatives are identified.
Mechanical feeders have problems with parts jamming. This is caused typically by parts which are very thin, out of tolerance or foreign. Some pieces have shapes which make them difficult, if not impossible, to orient. Some parts can be damaged by scratching against each other or against the orienting device. Large workpieces require very large feeders and substantial energy to be oriented. For vibratory feeders, vibrations may be conducted into other structures. Feed rates change with the number of parts in the feeder. Mechanical feeders often are excessively noisy. The cost and lead time to design, debug, support and changeover mechanical devices may be prohibitive. This is especially true for batch production applications. Even if a rejection mechanism is controlled by vision, most of the problems with mechanical feeders which have been cited remain.
Preserving the orientation of oriented parts is the most obvious way to avoid the problem of dealing with a bin of randomly oriented parts. This is particularly the case since commercially available industrial robots can load machines which are supplied in magazines or pallets. However, the preservations of orientation is often impractical or uneconomical. When different machines work at different rates, the rate of linked machines must be set by the slowest one. Buffer storage and line splitting and merging may be necessary to prevent the failure of one machine from stopping the entire line. If a robot is used to preserve orientation by unloading a piece from a machine rather than having the piece blown out with a blast of air and caught in a bin, the machine cycle may have to be extended and the thruput rate reduced. Other mechanisms to unload workpieces while preserving orientation may be expensive and space consuming. Pallets which are used to maintain the orientation of parts are susceptible to being upset during transfer from one workstation to another. Many machines are already set up to accomodate batch production and use stacks of bins for intermediate storage. Parts which come from vendors, long term storage or distant warehouses are usually shipped unoriented. The cost of storing or shipping oriented parts, due to low packing density, is usually prohibitive.
Due to the disadvantages of these three alternative approaches, robots with vision will be applied in the future to feeding workpieces. Their use will make it easy to change from one part to another for batch changes.
Various technical contributions have been made to the bin of parts problem. A data base of images of castings have been made available. A number of studies has been made which estimate the position and orientation of workpieces on flat surfaces, such as belt conveyors. The acquisition of a hanging part has been studied. Dihedrally tipped boxes which vibrate have been used to orient isolated parts. Overlapping parts, which are nearly horizontal, have been analyzed for position and orientation. Regions which belong to the same object have been identified in an image of a stack of blocks. The tasks of estimating the pose of pieces in the bin which were partially occluded and of acquiring only pieces which could be transported directly to a goal with a prespecified pose have been examined. Ellipses have been used to locate the circular vacuum cleaner filter which is on the "top" of a pile of similar parts. The parts of an assembly have been separated by a robot from a heap by grasping for protrusions which were located visually and by pushing with the hand at various levels. Heuristic visual methods for acquiring workpieces in bins have been described. Electromagnets have been dragged through bins to acquire billets.
Functional experimental robot systems employing special vacuum cup hands and simple vision algorithms have been described, "A Robot System which Feeds Workpieces from Bins into Machines", R. Kelley, J. Birk, D. Duncan, H. Martins and R. Tella, Proc. Ninth Intl. Symp. on Industrial Robots, Washington, D.C., pp. 339-355, Mar. 13-15, 1979; and "An Orienting Robot for Feeding Workpieces Stored in Bins", J. Birk, R. Kelley and H. Martins, IEEE Trans. Syst., Man, Cybern., Feb. 1981.
These systems had an integrated architecture which permitted all pieces to be transported to a goalsite with a prespecified orientation, regardless of which flat surface the vacuum cup acquired.